net: mana: Check if netdev/napi_alloc_frag returns single page

netdev/napi_alloc_frag() may fall back to single page which is smaller
than the requested size.
Add error checking to avoid memory overwritten.

Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>